Distillation process



April 24, 6 c. F. SILSBY 2,

DISTILLATION PROCESS Filed March 10, 1951 PULVERIZED TO BY- PRODUCT RECOVERY SYSTEM CARBONIZ/NG RETORT 1 PREHEATER GAS/F/CATION RETORT JNVENTOR.

c. FORBES SILSBY AGENI United States Patent 2,743,217 DISTILLATION PROCESS Charles Forbes Silsby, Scarsdale, N. Y., assignor to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York Application March 10, 1951, Serial No. 214,888

Claims. (Cl. 202-17) This invention relates to treatment in gaseous suspension of finely divided carbonaceous material. In certain of its aspects it deals with carbonization of carbonaceous material such as coal containing volatile or distillable components and with production therefrom of reactive gaseous products valuable for synthesis of chemicals.

The standard and well-known procedure for carbonizing coking coals, i. e., coals which become plastic and agglutinate when heated, has comprised heating a bed of the coal in a stationary bed, slot type oven at temperatures of say 1000 C. or somewhat higher for prolonged periods of time, e. g. 11-25 hours, while recovering volatile products driven out of the solid, then discontinuing the heating and pushing the mass of coke from the oven. The hot coke has then been cooled with water. If the coke were to be used as a source of synthesis gas or carbon monoxide as by reaction with steam or with carbon dioxide, it has been subjected as a bed to intermittent air blowing and steam blowing steps.

There are distinct and recognized disadvantages to processing of coal or of coke in stationary bed type ovens stemming in part from the inherently inefiicient intermittent operation, and in part from the poor conductivity of the coal or coke. Agglutinating characteristics of coal further emphasize the less desirable features of its treatment as a stationary bed. These disadvantages are reilected in high investment and operating costs for commercial installations of given capacity for coal charge and in sacrifices of quantity or quality of desired products recovered. Many prior efforts have been made to overcome these disadvantages.

My invention has among its objects improvement in carbonizing processes involving gaseous suspension of carbonizable carbonaceous material such as coal in hot gases and eflicient utilization of char so produced. Also among its objects-is processing of finely divided coal to yield valuable gaseous product streams containing carbon monoxide useful for synthesis into valuable'chemicals.

The attached drawing is a schematic representation of a plant in which certain aspects of my invention may be effected.

Referring to the drawing, reference number 1 indicates generally a vertical cylindrical carbonizing retort providing carbonizing chamber or zone 7 into which cold or relatively cool powdered carbonizable carbonaceous ma terial, such as coal, supplied by hopper 2 and feeding duct or conduit 3 containing a suitable feeding device 4, may be charged continuously and at controlled rate. To insure delivery of the coal to chamber 7 as freely flowing non-agglutinous solid, conduit 3 may be suitably cooled asby use of water cooling jacket 4'. ment of the coal in conduit 3 inert gas may be introduced into it as by pipe 5 and control valve 5'.

' :Upon admission to zone 7 the powdered material becomes dispersed in and settles through countercurrently flowing gases which have been heated to or somewhat above the desired coking or carbonization temperature To assist move- Patented Apr. 24, 1956 admitted to the lowerend of zone 7 by use of gas supply line 28, bustle manifold 8 and tuyeres 11 disposed at suitably spaced intervals around the shell of retort 1. The hot gases in supply line 28 are preferably inert chemically with respect to the coal and char at the conditions prevailing within reaction zone 7 and may be from any suitable source. They may,'for example, comprise products produced in retort 1 and returned, after separation of condensate therefrom, to line 28 by way of preheater 27 (of direct fired, pebble, or regenerative type) .and valve 31. They may be comprised wholly or in part of gas produced by subsequent oxidation of the char produced in retort 1, in which event such gas may be led to conduit 28 through valve 67 and line 66. Under certain conditions inert carbonization gas supplied to heater 27' may contain steam.

During dispersion and settling within zone 7 the coal is rapidly heated by and subjected to the distillation influences of the hot gases, and is maintained in suspension therein for a time sufficient to produce char, and tar and other distillate products. Distillate is led from the retort with the aforesaid hot gases through oiftake conduit 32, dust collector 33 and conduit 41 to distillate or by-product recovery (not shown).

The distillation residue or char settles into discharge hopper 12 and is continuously withdrawn therefrom by conduit 13 provided with suitable flow regulating and sealing means such as rotary valve 16. The char discharged through conduit 13 may be removed from the system and after suitable cooling be used for any desired purpose, for example, for powdered fuel or for mixture with suitable adhesive and molding into fuel briquettes. One valuable use for the char while still hot, after adjustment of its temperature if desirable, is to produce gases containing carbon monoxide, with or without hydrogen ;and/ or nitrogen, useful as fuel or as chemicals. Hot gases so produced may advantageously be utilized as or in the hot carbonization gas admitted to retort 1.

The last mentioned use of char is indicated diagrammatically in the attached drawing, according to which the hot char flowing through conduit 13 is fed continuously and directly into gasification retort 17' mounted therebelow for oxidation to gases containing carbon monoxide. 1

During passage between retorts 1 and 17 thechar,

, if it is at temperature below that desired-for its subse-' quent oxidation, may be heated to desired temperature by controlled combustion of part of it. To this end,

air or preferably air diluted with incombustible gases,

(to avoid the possibility of formation of explosive mixtures with gases which might migrate into conduit 13 from retorts 1 and/or- 17) may be admitted continuously to suitably sized conduit 13 by pipe 20 in predetermined quantity limited to effect burning of the predetermined portion, usually small, of the char passing through conduit 13 which effects the desired increase in its temperature. Resulting combustion products relatively rich in nitrogen and carbon dioxide may, if they are not objectionable in the products of the aforesaid retorts, mingle with such products. Prefer-ably, however, they are withdrawn from conduit 13 as by lines 20a and 20b to avoid such mingling. The withdrawn combustion products, or the carbon dioxide or nitrogen content of them, may be utilized as noncombustible diluent for air admitted to conduit 20. i

Hot char and such ash as may be formed during partial combustion in conduit 13 flows thence through inlet 18 of retort 17 (with the assistance if desired of gas admitted to the last named conduit by line 19) to be dispersed Within and to settle through gasification chamber or zone 21 countercurrently to upwardly moving oxidizing gas containing combined or free oxygen, suchas air, steam, carbon dioxide or any desired combination of these, while undergoing oxidation in contact therewith by reaction which on the overall rn'ay be either endothermic or exothermic. The selected oxidizing gas preheated to desired temperature, for example, to approximately 1000 C. or higher, admitted to the system by conduit 51 may as shown be led into the lower portion of retort 21 by bustle manifold 22 and tuyeres 24 disposed circumferentially about the lower end of retort 17.

Gaseous products of char conversion leaving reaction zone 21 by conduit 52, dust collector 53 and conduit 57 may be withdrawn from the system by line 62 for use as or in gaseous fuel or as raw materials for synthesis of alcohols, ammonia, hydrocarbonsor other compounds. However, 'since these products are at elevated temperature (often at least approximately that desired for carbonization in retort 1) part or all of them may be flowed, often without adjustment of temperature, through line 66 containing valve 67 and through line 28 into manifold 8 and tuyer'es 11 to provide at least part of the carbonization gases producing coal distillate and char. Sometimes it is desirable in order to maintain the hereinafter described preferred operating conditions within retort -1 to supplement such gases by additional gases preheated in heater 27.

Solid product from gasification retort 17, that is, ash resulting from combustion of the char, accumulates as a mass 26 in conical discharge hopper 23 in the bottom of retort 17 and is discharged continuously through line 47 containing flow control rotary valve 46 into a suitable receptacle for disposal such as car 48.

Finely divided carbonaceous solid and ash that may be carried out of retorts 1 and 17, respectively, with the gaseous products therefrom are preferably conducted through dust collectors 33 and 53 as aforesaid. These dust collectors may be in the form of Cottrell precipitators, or they may be cyclone type separators as shown diagrammatically. Solids, separated from the gas stream, after accumulating as piles 38 and 56 in the bottoms of these cyclones, are continuously withdrawn through lines 42 and 58, the former preferably leading to conduit 13, and the latter containing a rotary discharge valve 61.

Certain carbonaceous carbonizable materials, specifically most coals best suited for production of gaseous and other distillate products become plastic and 'agg-lutinous when subjected to elevated temperatures below terminal carbonization level. This phenomenon occurs for many such coals within the approximate temperature range of 400 to 500 6., certain individual coals exhibiting this property ove r ranges extending somewhat below or even somewhat above these limits. It hasthwarted commercial development or carbonization of finely divided coal in gaseous suspension, because it has resulted in formation of relatively large agglomerates having characteristically poor heat conductivity. These agglomerates cannot be carbonized to desired extent during the short time, a matter of seconds, that the coal is falling through the atmosphere of hot gases. Also, the agglutinate has adhered to retort walls forming encrustations'of agglomcrates of substantial proportions. I believe such encrustations have interfered with normal flow of gases and solids, the resulting disturbances tending to promote further-formation of agglomerates. V

In practice of the preferred aspect of my invention involving earboni'zation of carbonaceous material having agglutinating properties, e. g. coal, I admit 'a finely divided coal charge, say of 50 to 130 mesh size, while at substantially below 'agglomerating temperature, for exampleat atmospheric or other temperature below about 200* C., to the upper portion of a vertical carbonization zone. such as retort 1 of the drawings having lateral dimension as hereinafter indicated while simultaneously admitting to the bottom of that zone, a quantity of gases at or somewlfat 'above carbonizatio'n temperature sufficient to exert,

during its flow upwardly through said zone with produced coal gases, substantial but limited retarding effect on the fall of the carbonaceous solid therethrough. l have discovered that the difiiculties heretofore created by the agglomerating properties of the carbonaceous material are substantially eliminated if under these conditions the lateral dimension between material inlet and the nearest retort wall surface is such that none or substantially none of the solid material, while subject to the diffusing and incipient classification effects of the rising heating and producedgas, reaches the wall of the retort until it has passed through the plastic and agglutinous range, e. g. before its temperature is above its characteristic agglutinization range. I have found that this dimension should be of the order of five feet or more.

When operating in accordance with this aspect of my invention the bulk of the falling particles of coal (excepting permissibly the finest particles thereof) form a roughly outlined body of suspended solid, increasing in area as it falls so that the edges of it approach the retort walls; but the rate of such increase in area is such that the edges of the body do not contact, if at all, the next adjacent wall for a substantial distance, usually several feet, below the coal inlet. The minimum lateral dimension which will achieve the desired lack of contact or limited contact between carbonaceous solid and retort wall is dependent upon several factors, including size range and distribution of size of coal particles in the charge, relative ease or difficulty of carbonization of the charge to the degree desired, and the degree to which hot carbonizing gases retard fall of the carbonaceous solid. In retorts providing but a single centrally disposed inlet for coal such lateral dimension is reflected by the interior retort diameter indicated by dimension d of retort 1 of the accompanying drawing. I usually employ a dimension d of 10 feet or more. Dimension (1 is roughly correlatable with height of the carbonization zone, that is, the vertical distance 11 from the levels of coal and carbonizing gas admission indicated in retort 1 of the drawing. For many retort sizes and operating conditions conveniently applicable to plant operation distance d is one-half to two-thirds of distance 11.

In practice of this aspect of the invention it is preferred to employ such quantity of heated carbonizing gas that upward velocity of the gases within the retort results in residence time for the average particle within distance It of at least one and one-half times but no more than three times and ordinarily no more than twice the residence time of the average size particle of the charge settling through distance h in static air. Under more severe retarding conditions the tendency toward classification of the finest components of the charge with resultant lateral and upward movement of such components becomes too pronounced, leading to premature contact of the solid with adjacent surfaces and formation of agglomerates.

When the retarding conditions indicated are adhered to distances h of 20 feet or more are advisable to assure desired distillation and recovery of dry permanently hard freely flowable char. In installations where coals exhibiting agglutinating properties over a relatively wide temperature range or at high temperatures, or where charges varying substantially in this regard, may be anticipated, it is preferable to employ retorts providing distances h of 40 feet or more. When employing relatively short distances h as of the order of 20 to 30 feet, lateral distance d preferably is or approaches two-thirds of distance h; whereas with taller retorts, ratios of d to h at or approaching one-half will often prove satisfactory. It is obvious from the foregoing that for retorts other than those employing but a single coal irilet I contemplate providing minimum distances between a coal inlet and the next adjacentretort wall surface which are live to ten feet or more and which are one-fourth to one-third the height of the carbonization zo'ne.

Although I do not intend to limit my invention with particles of coal undergoing carbonization, I believe that I the bulk of the coal in the charge, including the larger particles, tends to concentrate in the central portion or core of the retort while the smallest particles, being more responsive to the classification effect of the rising gases, exhibit greater tendency than the larger ones to move laterally. If this be true it may well be that the smallest particles tending to concentrate within or beyond the outer portion of the main body of coal and char falling through the retort have contributed largely to the diificulties heretofore occasioned by agglutination. Whatever the cause of the prior difliculties, it appears that in practice of my preferred coal carbonization process, the substantial retarding effect of the flow of heated gas coupled with the large quantity of such gas necessary to achieve that effect in the large flow area of retorts characteristic of my process, result in rapid dispersion of the coal charge before it is heated to agglomerating temperature, and extremely rapid heating of the dispersed charge through the agglutinization range of temperature. The small portion of the charge representing the finest particles, which is almost inevitably elutriated from the main bulk of falling particles in a hindered settling system, also passes completely through the agglutinating stage before contacting retort or other surfaces. Thus, I realize inmy preferred process the full advantages of rapid heating of solid characteristic of finely divided particles suspended in hot gaseous medium, and maintain such advantages throughout the entire carbonization process while effecting carbonization of the entire charge.

The precise length of the path of coal particles moving toward the retort walls cannot be ascertained with accuracy because of the multiplicity of particle sizes within finely divided coal charges. However, due to the tendency of any gas introduced through pipe 5 and/or evolved gas formed almost immediate upon entry of the coal into the hot carbonization zone 7 to expand and thereupon disperse the entering coal stream, the paths followed by at least the preponderance of the coal particles lie within the volume generally enclosed by the outer broken lines shown within retort 1 of the drawing that is, within a confined generally paraboloidal space. With coals having low agglutination temperature it is sometimes desirable to accelerate their diffusion to limited extent by means of a suitable distributing device (not shown) disposed at the lower end of the charge inlet 6. The use of such a device tends to modify somewhat the shape of the paraboloid by broadening of its base, without, however, adversely aflecting the desired course of the carbonization process. :2.

To effect complete or substantially complete carbonization of the coal, it is preferred that the hot gas supplied be at temperature of 800 C. or higher,.for example, of

the order of 1000 C. Under my preferred carboniza tion. conditions, the short time of residence of the coal and produced gases at carbonization temperature results in high recovery, even at high coking temperatures, of valuable distillate tars and the like. However, in. order to achieve highest possible recovery of tars, if that be desired, it is preferred to resort to somewhat lower gas temperatures, say 600 C. to 800 C., which are higher, preferably by at least 100 C., than the agglutinating temperature of the coal charge. In either event operation effected according to this aspect of my invention is substantially free of the difiiculties occasioned by the agglutinating properties of the coal, and the char recovered is finely divided and free flowing.

The carbonization gases employed are as above indicated preferably inert, that is, unreactive with respect to the coal or char under the conditions prevailing within the carbonization zone. As above indicated, fixed gases from the carbonization step itself and gases obtained by oxidation of the resulting char are well suited for this purpose. Another inert gas which may be used alone or in admixture with the aforesaid gases, particularly at relatively low carbonization temperature, is steam. If theflcarbonization temperature is within the range favoring a high degree of dissociation of steam, it is preferred that the quantity of steam in the carbonization gases, if used, be kept sufiiciently low that such dissociation does not unduly or substantially reduce the temperature prevailing within carbonization zone.

In operation of the gasification stage when the oxidiz- 'ing medium for the char is one containing combined oxygen and is dissociated during the reaction to produce overall endothermic effect, as is the case with carbon dioxide and water, it is preferred that the char as admitted to the gasification stage as in retort 17 be at temperature at least as high as or higher than the appro priate dissociation temperature. To that end'my invention in certain of its aspects contemplates as above described increasing the sensible heat content of the char during its passage between the retorts by controlled combustion of part of it by admission to it of combustion supporting medium. By properly regulating the quantity of oxygen'so supplied the temperature of the char may be raised from desired distillation temperature, whether relatively low or relatively high, to any extent consistent with supplying the desired quantity of sensible heat for thesubsequent dissociation-oxidation reaction, for example to 1200 C. to 1500" C.'or higher. The supply f heat for the strongly overall endotherm is thus realized under very favorable and economical conditions for pro- .tarice to the nearest retort wall is provided.

dosing and exchanging heat and without destruction of desired combustible gaseous lay-products such as hydrogen and carbon monoxide. However, if, for limiting reasons such as the maximum temperature which the equipment employed considering its design can withstand safely, it is desirable or necessary to augment or substitute sensible heat content of char so produced, heat for supporting the endotherm may be supplied by combusion of a regulated quantity of the contents of the gasification zone by including with disassociatable oxidizing medium free oxygen as such or in air.

When the gasification step to be employed involves an overall exothermic reaction, as is the case when free oxygen is the preponderating active component of the gaseous reactant, it is sometimes advisable, in order to maintain the desired heat balance and reaction temperatures within the gasification zone, to cool the char before its admission to the gasification retort. This may be done by admission into conduit 13 through pipe 20 of cool inert gases, for example, cool gases produced by the carbonization step and recycled from recovery equipment therefor.

Since the moisture content of the material introduced into the carbonizing zone 7 will have some bearing on the heat required to complete carbonization, it is desirable to maintain the moisture content of the coal charge as low as feasible, preferably not above 23% by weight.

The drawing and foregoing description set forth an embodiment of the invention according to which puiverized coal is introduced vertically downwardly into carbonizing zone 7 by a single central inlet. it will be apparent that a number of such inlets disposed vertically or angularly may be employed, providing the proper dis- If'a number of inlets are employed they should be so spaced one from the other that solids discharged from them will not intermingle before passing through the agglutinous stage.

The hot pulverized char fed to the gasification retort 17 being free or substantially so of agglutinating properties, the conditions and mode of carrying out the gasifica tion reaction with respect to physical dimensions of the reaction zone are less critical than the conditions for carrying out the carbonization reaction. Instead of cans ing the particles of char to settle through a rising current of air, they may be suspended in the oxidizing gas to move cocurrently through the gasification zone or to form therein a fluidized bed. Other modes of procedure will become apparent to those skilled in the art, important considerations being maintaining contact between pulverized char and oxidizing gas for a time sufficient to effect substantially complete oxidation of the carbon in the char.

I claim:

1. The method of carbonizing finely divided coal subject to agglutination while being heated toward coking temperature comprising introducing a stream of such coal at temperature of at least 100 C. below its characteristic agglutinating level into a vertical carbonization zone at a point in the upper portion thereof spaced from the next adjacent side walls by at least five feet, causing said coal to settle through said zone countercurrently to and suspended in hot inert carbonization gas flowing therethrough for time sufficient to effect carbonization thereof by the action of said gas, admitting the latter to a lower level of said zone at coking ternperature of at least 800 C. and in quantity sufiicien't that as the result of upward velocity of gases within said zone the settling time of the finely divided solid therethrough is at least one and one-half but no more than three times its settling time through static air, and maintaining said suspended coal out of contact with wall surfaces of said zone until it has been heated to and through the range of temperature within which it is agglutinous.

2. The method of carbonizing coal which is subject to agglutination upon being heated toward coking temperature comprising the steps of admitting a stream of such coal in finely divided condition and at temperature substantially below its agglutinating level to the upper portion of a vertical carbonization zone within which it becomes dispersed in and settles countercurrently through upwardly moving hot inert carbonization gases to be subjected as the result of contact with said gases to distillation and coking, supplying said gases to a lower portion of said zone heated to coking temperature of at least 800 C. and at such rate that the settling time of said coal therethrough is one and one-half to two times the settling time thereof through static air, removing said hot gas and distillation products from the upper end of said zone, maintaining said coal suspended in said gas for time sufiicient to effect coking of said coal, and caus ing said coal to follow a path during at least a portion of such contact such that the main body thereof remains out of contact with wall surfaces until the coal has been heated to temperature above its agglutination range.

3. The method of carbonizing finely divided coal which is subject to agglutination upon being heated toward col;- ing temperature involving flowing the coal in suspension in hot inert gases downwardly through a vertical carbonization zone countercurrently to and in suspension in such gases preheated to carbonization temperature of at least 800 C. for time sufficient to produce distillate product and dry freely flowing char, comprising the steps of controlling the quantity of such gases admitted to said zone to achieve gas velocity therein which by its retarding effect on the falling coal produces a residence time thereof in suspension equal to one and one-half to three times its residence time settling through said zone in static air, and maintaining said coal substantially out of contact with walls of said zone until the distillation and coking thereof is substantially complete.

4. The method of carbonizing finely divided soli carbonaceous material subject to agglutination while being heated toward coking temperature comprising introducing a stream of such material at a temperature of at least C. below its characteristic agglutinating level into a vertical carbonization zone at a point in the upper portion thereof spaced from the next adjacent side Walls by at least five feet, causing said material to settle down-. wardly in dispersed form countercurrently to upwardly rising hot inert carbonization gas supplied to said zone at coking temperature of at least 800 C. and in quantity sufficient that as a result of upward velocity of gases.

within said zone the settling time of the finely divided material therethrough is at least one and one-half times but no more than three times its settling time through static air, and maintaining the settling material in dispersed form out of contact with wall surfaces of said zone for time sufiicient to effect heating thereof to and through the range of temperature at which it is subject to agglutination.

5. The method .of carbonizing finely divided solid carbonaceous material subject to agglutination while bing heated toward coking temperature comprising introducing a stream of such material at a temperature at least 100 C. below its characteristic agglntinating level into a vertical carbonizaticn zone at an upper point thereof spaced from the next adjacent side walls by at least five feet to fall through said zone, simultaneously introducing into said zone at a lower level hot inert carbonization gas heated to a temperature of at least 800 C. to flow upwardly therethrough in quantity sufficient to retard the downward movement of said material that as a result of upward velocity of gases within said zone the settling time of the finely divided material therethrough is at least one and one-half times but no more than three times its settling time through static air, maintaining said gas and material in counter-current flow relation and said material in dispersed suspension within said zone for time sufiicient to effect distillation and coking of said material, and maintaining said material out of contact with wall surfaces of said zone until the coal has been heated to a temperature above the range within which it is agglutinous.

References Cited in the file of this patent UNITED STATES PATENTS 706,443 Naef Aug. 5, 1902 899,690 Benner Sept. 29, 1908 1,473,491 Manning Nov. 6, 1923 1,775,323 Runge Sept. 9, 1930 1,782,556 White Nov. 25, 1930 1,783,982 Runge Dec. 9, 1930 1,797,796 Rungc Mar. '24, 1931 1,866,399 Baufre July 5, 1932 2,366,055 Rollman Dec. 26, .1944 2,560,403 Arveson July 10, 2,605,215 Coghlan July 29, 1952 2,623,815 Roetheli Dec. 30, 1952 2,626,235 Wilson Jan. 20, l953 FOREIGN PATENTS 242,622 Great Britain Mar. ll, 1926 242,623 Great Britain Q. Dec. 2, 1926 286,404 Great Britain Mar. 8, 1928 532,055 Great Britain Nov. 4, 1946 

1. THE METHOD OF CARBONIZING FINELY DIVIDED COAL SUBJECT TO AGGLUTINATION WHILE BEING HEATED TOWARD COKING TEMPERATURE COMPRISING INTRODUCING A STREAM OF SUCH COAL AT TEMPERATURE OF AT LEAST 100* C. BELOW ITS CHARACTERISTIC AGGLUTINATING LEVEL INTO A VERTICAL CARBONIZATION ZONE AT A POINT IN THE UPPER PORTION THEREOF SPACED FROM THE NEXT ADJACENT SIDE WALLS BY AT LEAST FIVE FEET, CAUSING SAID COAL TO SETTLE THROUGH SAID ZONE COUNTERCURRENTLY TO AND SUSPENDED IN HOT INERT CARBONIZATION GAS FLOWING THERETHROUGH FOR TIME SUFFICIENT TO EFFECT CARBONIZATION THEREOF BY THE ACTION OF SAID GAS, ADMITTING THE LATTER TO A LOWER LEVEL OF SAID ZONE AT COKING TEMPERATURE OF AT LEAST 800* C. AND IN QUANTITY SUFFICIENT THAT AS THE RESULT OF UPWARD VELOCITY OF GASES WITHIN SAID ZONE THE SETTLING TIME OF THE FINELY DIVIDED SOLID THERETHROUGH IS AT LEAST ONE AND ONE-HALF BUT NO MORE THAN THREE TIMES ITS SETTLING TIME THROUGH STATIC AIR, AND MAINTAINING SAID SUSPENDED COAL OUT OF CONTACT WITH WALL SURFACES OF SAID ZONE UNTIL IT HAS BEEN HEATED TO AND THROUGH THE RANGE OF TEMPERATURE WITHIN WHICH IT IS AGGLUTINOUS. 